1. Field of the Invention
The present invention relates to methods for forming a dielectric thin film, and more particularly to a method for forming a dielectric thin film using a solution containing starting materials.
2. Description of the Related Art
In order to form a dielectric thin film, using a material solution, the following methods have been applied:
(1) Liquid Source CVD (LSCVD): a solution in which organic metal materials are uniformly dissolved is vaporized to react with the surface of a substrate, thus forming an oxide thin film.
(2) Metal Organic Deposition (MOD): a solution in which organic metal materials are uniformly dissolved is applied, dried, calcined, and fired to form an oxide thin film.
(3) A spray method: a material solution is sprayed onto the surface of a substrate, followed by drying and firing to form an oxide thin film (for example, a solution is sprayed onto a substrate by ultrasonic waves, and is subsequently heated to dry and heat-treated to form a thin film, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 9-213643).
However, the LSCVD method described as method (1) above, requires the use of expensive compounds having a high vapor pressure as raw materials because the raw materials must be in a gas form to be applied onto a substrate. Thus, LSCVD has disadvantage in that it is very expensive.
On the other hand, the MOD method described as method (2), and the spray method, described as method (3), have no vapor pressure requirement because a solution containing raw materials is applied in a liquid form onto a substrate. However, the vaporized solvent passes through the deposited film during drying to form cavities which are likely to cause a short circuit between electrodes disposed on both surfaces of the resulting film if the film thickness is small. It is thus difficult to form a reliable dielectric thin film.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method for forming a dielectric thin film having high reliability, and provide a dielectric thin film formed by the novel method.
According to a preferred embodiment of the present invention, a method for forming a dielectric thin film includes the steps of (a) spraying a material solution including a starting material and a solvent onto a heated substrate under a reduced pressure by a two-fluid technique using an inert gas to deposit a thin film, and (b) subjecting the thin film to heat treatment in an oxidizing atmosphere.
By performing this film deposition step, the steps of drying and calcining the deposited film, which are necessary in known methods, such as the MOD method and the spray method, can be eliminated. Thus, the vaporization of the solvent from the deposited film is prevented and the occurrence of cavities (loopholes of solvent vapor) is reliably prevented in the thin film. Consequently, a short circuit between electrodes disposed on both surfaces of the film is reliably prevented. Thus, a reliable dielectric thin film can be provided.
In the method of preferred embodiments of the present invention, characteristics of the resulting dielectric thin film depend on conditions for film deposition before heat treatment rather than on heat treatment conditions. Therefore, by selecting the conditions for film deposition, a reliable dielectric thin film can be efficiently formed.
Also, by heat-treating the deposited thin film in an oxidizing atmosphere, organic constituents of organic metal compounds in the material solution are surely burned and removed. Thus, a reliable, precise dielectric thin film can be provided.
More specifically, in the method of preferred embodiments of the present invention, the solvent in the sprayed material solution is rapidly vaporized to be removed immediately after reaching the surface of the heated substrate. Thus, a film hardly including any of the solvent can be deposited on the substrate.
Thus, the vaporization of the solvent from the deposited film can be prevented and, thus, the occurrence of cavities in the resulting thin film is prevented. Consequently, a reliable dielectric thin film can be achieved in which short circuits do not occur between electrodes, even if the electrodes are disposed on both surfaces of the film.
Preferably, the material solution is sprayed under conditions that a major portion of the solvent vaporize immediately soon after the solvent reaches a surface of the substrate.
By spraying the material solution under conditions allowing a major portion of the solvent to vaporize immediately, the vaporization of the solvent is prevented in the deposited thin film and the occurrence of cavities in the resulting thin film is reliably prevented and minimized. Thus, a reliable dielectric thin film can be provided.
Preferably, the film deposition step and the heat treatment step are performed two or more times.
By subjecting one substrate to the film deposition and heat treatment steps two or more times, a reliable, precise dielectric thin film in which short circuits do not occur can be provided.
If the material solution includes an organic metal compound, very few cavities or voids are inevitably formed in the thin film when the organic constituents of the organic metal compound are removed, even if the solvent can be efficiently removed in the film deposition step. However, by repeating the film deposition step and the heat treatment step, the cavities or voids, which are likely to cause short circuits, are filled, so that the occurrence of short circuits can be reliably prevented.
Preferably, the material solution includes at least one metallic element in a total concentration of approximately 0.01 mol/L or less.
By using a material solution including approximately 0.01 mol/L or less of the metallic element, the size of lumps formed by the solidification of drops of the sprayed material solution can be reduced and, thus, a dielectric thin film having a uniform, small thickness can be achieved.
Since the substrate is heated in the method of preferred embodiments of the present invention, a material solution having an excessively high concentration is likely to result in lumps having a grain size as large as several micrometers. This makes it difficult to form a dielectric thin film having a thickness as small as a submicron. Also, the surface morphology is significantly degraded due to the resulting rough surface.
The inventors conducted an experiment to determine a suitable concentration of the material solution for forming a film having an adequately small thickness. As a result, it has been discovered and shown that, by using a material solution having a metallic element concentration of approximately 0.01 mol/L or less, the size of lumps formed on the surface of the substrate can be reduced so that the thickness of the dielectric thin film becomes uniform and small. Therefore, it is preferable to use a material solution having a metallic element concentration of approximately 0.01 mol/L or less.
According to another preferred embodiment of the present invention, a method for forming a dielectric thin film includes the steps of (a) spraying a material solution including a starting material and a solvent onto a heated substrate under a reduced pressure by a two-fluid technique using an inert gas to deposit a thin film, (b) stopping the supply of the material solution and vaporizing the solvent remaining in the thin film, and (c) subjecting the thin film to heat treatment in an oxidizing atmosphere.
The heat treatment step is performed after the film deposition step and the solvent vaporization step is preferably repeated once or more times. Also, the material solution is supplied at a rate that is greater than the vaporization rate of the solvent in the film deposited on the substrate.
By supplying the material solution at a rate that is greater than the vaporization rate of the solvent in the film to deposit a thin film, then vaporizing the solvent remaining in the film with the material solvent not supplied, and heat-treating the film in an oxidizing atmosphere, the vaporization of the solvent is prevented in the deposited thin film and, thus, the occurrence of cavities is reliably prevented in the resulting thin film. Thus, a reliable dielectric thin film is provided.
Although the method includes the solvent vaporization step after the film deposition step, the solvent is proactively vaporized by heating the substrate during film deposition. The deposited film is gradually dried from the substrate side while the solvent is vaporized. Thus, the occurrence of cavities can be prevented in the solvent vaporization step effectively, and consequently, a reliable dielectric thin film is provided.
In this method, the supply rate of the material solution can be increased to increase the speed of film formation.
The speed of film formation in this method is a few times to several tens of times higher than that of other processes of preferred embodiments of the present invention.
Also, the material solution can be efficiently adhered to the substrate to increase raw material efficiency.
Preferably, the film deposition step is performed at a pressure of approximately 13.3 kPa (100 Torr) or less.
By setting the pressure in the film deposition step at approximately 13.3 kPa or less, a reliable dielectric thin film which has excellent characteristics and does not exhibit short circuits even if the thickness is small is provided.
Preferably, the thickness of a film deposited at one time is approximately 50 nm or less.
By setting the thickness of a film deposited at one time to be approximately 50 nm or less, cavities are surely prevented and thus, a reliable dielectric thin film can be achieved. If the thickness deposited at one time becomes larger than approximately 50 nm, the solvent vaporized from the film easily passes through the thin film to undesirably form cavities.
Preferably, the solvent vaporization step is performed at a pressure lower than the pressure in the film deposition step.
By setting the pressure in the solvent vaporization step lower than the film deposition step, the solvent can be efficiently vaporized and removed.
Preferably, the substrate is heated to a temperature in the range of about 100xc2x0 C. to about 300xc2x0 C.
By heating the substrate to about 100xc2x0 C. to about 300xc2x0 C. in the film deposition step, a reliable dielectric thin film can be achieved which has excellent characteristics and does not exhibit short circuits even if the thickness is small.
A temperature of higher than about 300xc2x0 C. leads to the decomposition of the solvent, thus allowing the thin film to include a large amount of decomposition products. The decomposition products remain in the thin film even after heat treatment, consequently increasing leak current in comparison with in the case of a temperature of about 300xc2x0 C. or less. Accordingly, it is preferable to set the temperature for heating the substrate at about 300xc2x0 C. or less.
The temperature for heating the substrate can be reduced to about 100xc2x0 C. without problems. However, a temperature of lower than about 100xc2x0 C. reduces the vaporization rate of the solvent of the material solution to allow a large amount of the solvent to remain in the thin film. This significantly degrades the surface morphology and the resulting dielectric thin film is liable to exhibit a short circuit. Accordingly, it is preferable to set the temperature for heating the substrate in the range of about 100xc2x0 C. to about 300xc2x0 C.
Preferably, the heat treatment is performed at a temperature of about 500xc2x0 C. or more.
By setting the temperature for heat treatment at about 500xc2x0 C. or more, organic constituents of an organic metal compound included in the material solution can be surely burned and removed. Thus, a reliable, precise dielectric thin film can be achieved.
Preferably, the material solution includes (a) titanium and barium; or (b) titanium and strontium.
By using a material solution including titanium and barium; or titanium and strontium, a reliable barium titanate or strontium titanate dielectric thin film having various uses can be efficiently formed.
Other preferred embodiments of the present invention are directed to a dielectric thin film formed by the method according to the preferred embodiments described above. The dielectric thin film of such a preferred embodiment preferably has (a) crystallinity; (b) a thickness of about 200 nm or less; and (c) a relative dielectric constant of about 250 or more.
The dielectric thin film has high reliability and can be suitably used for multilayer ceramic electronic components such as monolithic ceramic capacitors.
The dielectric thin film may be formed by repeating the film deposition and heat treatment steps one or more times.
By subjecting one substrate to the film deposition and heat treatment steps two or more times, the resulting dielectric thin film has high reliability and does not exhibit a short circuit even if electrodes are disposed on both surfaces thereof. Thus, the dielectric thin film can be suitably used for multilayer ceramic electronic components such as monolithic ceramic capacitors.
Also, other preferred embodiments of the present invention are directed to another dielectric thin film formed by other methods according to preferred embodiments of the present invention. This dielectric thin film according to another preferred embodiment preferably has (a) crystallinity; and (b) a relative dielectric constant of about 250 or more.
The dielectric thin film has high reliability and does not exhibit a short circuit even if electrodes are disposed on both surfaces thereof. Thus, the dielectric thin film can be suitably used for multilayer ceramic electronic components such as monolithic ceramic capacitors.
Preferably, the dielectric thin film includes (a) titanium and barium; or (b) titanium and strontium.
The resulting barium titanate or strontium titanate dielectric thin film has high reliability and various uses.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.